1. Field of the Invention
The present invention relates to an organic electroluminescent device and manufacturing method thereof.
2. Description of the Background Art
The organic electroluminescent device (hereinafter referred to as “organic EL device”) is expected as a new type self light emitting device. The organic EL device has a layered structure including a carrier transport layer (electron or hole transport layer) between a hole injection electrode and an electron injection electrode, and a light emitting layer.
An electrode material having a large work function such as gold or ITO (Indium-Tin Oxide) is used for the hole injection electrode, while an electrode material having a small work function such as Mg (magnesium) or Li (lithium) is used for the electron injection electrode.
An organic material is used for the hole transport layer, the light emitting layer and the electron transport layer. A material having the characteristic of p-type semiconductor is used for the hole transport layer, while a material having the characteristic of n-type semiconductor is used for the electron transport layer. The light emitting layer also has carrier transport capability such as electron or hole transport capability, and is made of an organic material emitting fluorescent light or phosphorescent light.
Note that depending upon organic materials to be used, these functional layers such as the hole transport layer, electron transport layer and light emitting layer may each include a plurality of layers or some of them may not be provided at all.
In order to improve operating stability of the organic EL device, insertion of a CuPc (copper phthalocyanine) layer between a hole injection electrode and a hole transport layer has been proposed. However, the insertion of the CuPc layer between the hole injection electrode and hole transport layer has a disadvantage of increasing the initial drive voltage (refer to, for instance, JP-2000-150171-A).
In addition, the organic EL device may be deteriorated when exposed to external ultraviolet irradiation, depending upon the materials. This phenomenon will, hereinafter, be called “light deterioration”. The light deterioration lowers the luminance of an organic EL device. For this reason, it is necessary to increase the drive voltage with time in order to keep the luminance of the organic EL device constant.